Naphthenic acid fraction components (NAFCs) are thought to be a primary agent of toxicity in oil sands process waters (OSPWs) produced by industrial activity in Canada's Athabasca oil sands. They are a complex, poorly characterized mixture of compounds whose mechanisms of toxicity are not well understood. In this work, it was discovered that the unicellular green algae Chlamydomonas reinhardtii are much more tolerant of NAFCs than predicted based on comparison to Chlamydomonas spp. isolated from the OSPW tailings ponds, with exponential growth occurring at 100 mg L− 1 NAFC. Two cell wall mutants of C. reinhardtii exhibited greater tolerance to NAFC exposure. NAFC exposure induced changes in growth form and morphology were most pronounced in wild-type cells. Confocal scanning laser microscopy and Fourier-transform infrared spectromicroscopy indicated changes in cell wall surface proteins and their confirmation after exposure to NAFCs. Such alterations of cell wall proteins are consistent with the effects of surfactants on green algae, and indicate a possible role for classic naphthenic acids in the NAFC mixture to cause surfactant-mediated toxicity. The much greater tolerance to NAFCs under laboratory conditions indicates the likelihood that NAFCs do not act alone as agents of toxicity in algae such as C. reinhardtii, rather they seem to act in combination with other environmental factors to potentiate toxicity.